In general, Chemical Mechanical Polishing (CMP) equipment is important semiconductor equipment employed to polish the surface of a wafer. CMP equipment is generally comprised of a polishing apparatus and a loading device. The polishing apparatus includes a platen on which a polishing pad is attached, a slurry supply that supplies a slurry for chemical polishing to the polishing pad, a spindle which grasps and rotates the wafer in contact with the polishing pad by means of a polishing carrier located above the polishing pad, thereby physically polishing the wafer, and so on. The loading device delivers the wafer, which is transferred from a wafer cassette by a robot arm, to a head of the polishing carrier so as to be able to load/unload the wafer on/from the polishing carrier head.
In the semiconductor process, it is important to control any process, for instance a polishing process of a processing target such as a wafer by monitoring its progress in real time and terminating it at a proper point of time, which is called detection of a process end point or a polishing end point. There is a device which is adapted to detect this polishing end point and transmit a signal to a process module controller to complete the process. This device is called an end point detector (EPD).
In particular, in the case of a CMP process using CMP equipment, the polishing end point is detected by measuring the thickness of the wafer before and after polishing. To this end, an in-line metrological thickness measurement technique using an optical system is typically applied. The polishing end point in a semiconductor wafer surface polishing process based on the in-line metrological thickness measurement technique can be detected by emitting light from a light source, reflecting the light on the surface of the wafer before and after polishing, and allowing a photo-detector (or a probe assembly) to receive the reflected light and measure change in interference of the received light. In this manner, information on a removal rate can be obtained and applied when a subsequent wafer is polished, thus enabling more precise polishing control.
This wafer polishing process may entail polishing only one wafer at a time, transferred by the loading device on a single platen. But, in most cases, the process entails sequentially polishing a plurality of wafers transferred by a plurality of loading devices, installed around the polishing apparatus, on a plurality of platens arranged adjacent to each other, which is called a multi-step polishing process.
Concrete examples of conventional methods and apparatuses for detecting the polishing end point by employing the optical thickness measurement technique of the semiconductor wafer described above are disclosed in Korean Patent Application Nos. 10-2003-0018522 and 10-2003-0027043, filed by the present applicant.
Korean Patent Application No. 10-2003-0018522 discloses technology for detecting change in a layer s thickness by using an interference phenomenon without depending on the intensity of reflected light as did previous technology, when the end point of a process of polishing a layer on a wafer to a predetermined thickness is detected by an optical system. Using this technology, the end point of the polishing process can be precisely detected.
Korean Patent Application No. 10-2003-0027043 discloses a technical configuration in which a probe assembly is installed in a platen of CMP equipment so that a tip of the probe assembly can approach the surface of a wafer in order to allow polishing information to be recognized in real time while the surface of the wafer is polished.
According to the concrete embodiments disclosed in the specifications of these prior patent applications, a transmission window is formed by boring a hole in a polishing pad and covering it with a light-transmission protective cap, light is directly applied onto a wafer through the transmission window, and a change in layer thickness is detected based on change in a property of the reflected light. In other words, the conventional technology detects the polishing end point as a point of time when a unique change is detected in a process of performing multi-step correction on digital data obtained through an optical sensor (probe), and stops the polishing process at the point of time.
In the conventional art described above, the probe assembly for detecting the light is mounted in the platen of the polishing equipment, the polishing process is performed, change in thickness of the layer being polished can be simultaneously tracked in real time, and thereby the polishing end point can be detected. Here, the light is applied at designated locations on the wafer, and waveform signals of the reflected light according to the layer thickness of the wafer are analyzed to obtain the thickness information. Here, the polishing end point can be instructed by a command system stopping the polishing process at a specified peak or valley of the waveform of the reflected light.
However, the conventional method and apparatus for detecting the end point have the following problems.
First, because the surface (pattern surface) of the wafer is polished chemically and mechanically by the CMP equipment, a large quantity of noise and unnecessary data are mixed in with the data obtained by detecting the surface of the wafer. Thus, compared to previous methods, it is necessary to process a large volume of complicated data. According to a pattern type, end point detection precision may be lowered.
Second, in the structure where the transmission window is formed on the polishing pad, change in reflection characteristics (distortion phenomenon) such as refraction is caused by water or slurry existing between the wafer and the tip of the probe during the surface polishing process. Also, transmission and reflection performance may be lowered by damage to the surface of the transmission window while the transmission window (light-transmission protective cap) formed on the polishing pad causes physical friction with the polishing carrier and conditioner. As a result, the end point detection precision can be lowered. Moreover, the surface of the wafer may be scratched or unevenly polished by the transmission window, thereby causing defects and reducing the lifespan of the polishing pad.
Third, an error may occur in the measurement for detecting the end point, because the light transmittance ratio varies depending on distance between the wafer and the probe or a surface state of the probe protector (light-transmission protective cap) covering its end so as to protect the probe. In order to compensate for this measurement error, a separate automatic gain control (AGC) process is required, which makes the whole polishing processes complicated.
Fourth, as both the wafer and the probe assembly are rotated in the wafer surface polishing process should first be synchronized. This may pose an obstacle to simple and convenient equipment operation.
Meanwhile, the wafer controlled to be polished up to a specified polishing end point by the above-described end point detecting method and apparatus is transferred into a wafer station installed at one side of the polishing apparatus before and after polishing. It is measured whether a pre-polishing state of the layer of the wafer is normal, or whether the layer of the wafer is polished to a desired proper thickness, thus testing for processing defects on the wafer. Here, a thickness measurement detection technique is employed. In the case of a continuous single-step polishing process of numerous wafers, the thickness measurement detection technique is for extracting polishing information about a previously input wafer to provide information that can be reflected in a polishing process of a subsequently input wafer. And, in the case of a multi-step polishing process of a single wafer, the thickness measurement detection technique is for extracting polishing information about an input wafer to provide information that can be reflected in a subsequent polishing process of the wafer. An optical in-line metrological thickness measurement detection apparatus similar to the end point detection apparatus is used to perform the thickness measurement detection technique.
More specifically, the conventional in-line metrological wafer layer thickness measurement technique is carried out in a wafer station of the polishing equipment, and generally performed on the wafer before beginning a polishing process or after completing polishing and cleaning processes. Light is applied at numerous designated locations on the wafer, and correlation between the waveform signal of the reflected light and the layer thickness of the wafer is analyzed and converted into information, so that information on the layer thickness can be obtained. Thus, it can be determined whether or not the processed wafer has been polished normally.
However, due to characteristics of the method and apparatus, the conventional in-line metrological wafer layer thickness measurement technique can only obtain information related to removal rate, such as information on layer thickness, after all polishing processes of the previously input wafer have been completed. Hence, there is a corresponding delay in obtaining the information about the polishing processes, and consequently, it is inevitable that the value and availability of the obtained information are correspondingly lowered. Also, the layer thickness of the wafer can only be measured in a separate wafer station for measuring the layer thickness after the wafer is transferred up to a position of the optical system, so that the overall polishing process is delayed. Further, the wafer station increases the size of the equipment, and thus the arrangement and space utilization are lowered.